1. Field of the Invention
This invention is directed generally to a heat pipe apparatus for heating applications, more particularly to a heat pipe apparatus adapted to be employed as a heating appliance of natural convection type.
2. Description of the Prior Art
There have been proposed heater units of the type adapted for use in natural convection heating, such as shown in FIG. 1, wherein an elongated rod-like electric heat source 6, for example a sheathed electric heater composed of a heating coil and an insulating covering, is provided with a series of spaced fins 7 attached to the outer surface thereof. In the above heater unit, it is most desirable for increasing heat transfer efficiency to have an intimate contact between the entire periphery of the outer surface of the heat source 6 and the fins 7. However, a hole in each fin 7 is required for facilitating the operation of inserting the heat source 6 therethrough and having a diameter slightly larger than the outer diameter of the heat source. This results in a loose contact or gap between the heat source 6 and fins 7. To fill the gap after inserting the heat source 6, one alternative is to enlarge the diameter of the heat source 6 to attain intimate contact, but it is unlikely that the heat source of this type will have such self deforming ability. Therefore, the above heater units will suffer from the gap between the heat source and the fins, which will give rise to poor heat transfer efficiency from the heat source to the surrounding air.
Further, almost all of the conventional heat sources are seen to have a considerable temperature gradient along the length thereof, thus it often occurs in such heater units that the fins are heated to different temperatures depending on their location along the length of the heat source, which will cause lowering of the capacity of the heater unit in spite of it being required to operate with maximum efficiency. Another disadvantage resulting from the construction of the above heater unit is that the fins may be sometimes overheated to such an extent that mild heating performance is not available.
In the meanwhile, the above heater units are designed to be used normally as incorporated in a housing 1, as shown in FIG. 4. In actual situations, it will frequently be required to increase the heat transfer capacity of the heater units depending upon the space in which they are installed. For increasing the amount of heat to be released, adding an extra number of fins to the heat source may be the first thought, but mere addition of fins of fixed length to the heat source proves to be less effective by the reason that fins spaced too closely together will certainly impede the upward flow of air through the fins so as to reduce the amount of heat released from the fins to the air. Thus, there remains the choice of either elongating the heat source so as to increase the number of fins employed while maintaining the clearance between the adjacent fins at an optimum value or to add another heater unit with the same heat source so as to increase the total number of fins. However, the former measure has a disadvantage in that the old heater unit, replaced by a new one of greater heat transfer capacity, is abandoned and wasted, and the latter measure has a disadvantage in that the addition of extra heater units incurs increased equipment cost. This results in higher operating costs.